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September 201991. Jahrgang

CITAH 91 (9)1205-1352 (2019)ISSN 0009-286 X

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9 | 2019Schwerpunkte:EnergiespeicherStrömungenSystemverfahrens-technik

Herausgeber:DECHEMAGDChVDI·GVC

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Tube Swirler Inserts – Development StatusMartin Effert* and Jan Bruckner

DOI: 10.1002/cite.201800143

Tube swirler inserts were developed for the evaporator heating surfaces of once-through boilers with a firing system. The

improvement of the internal heat transfer has been shown in extensive measurements. Application of the swirler inserts is

not limited to the power industry. Any heat transmission process might benefit from the new technology, especially in case

of two-phase flow. The inserts can be introduced into smooth tubes of any steel grade.

Keywords: Benson boiler, Coils, Heat transfer improvement, Rifled tubing, Twisted tapes, Two phase flow

Received: August 30, 2018; revised: December 14, 2018; accepted: June 27, 2019

1 Motivation for Development

Various boiler concepts were investigated in subprojects ofthe EU-sponsored Thermie and AD700 R&D projectsheaded by VGB PowerTech e.V. [1]. The objective was toconstruct a supercritical coal-fired demonstration powerplant that would achieve an efficiency rate of up to 55 % atapprox. 400 MW.

In modern supercritical boilers, competition is primarilybetween the evaporator concepts with spiral tubing withsmooth tubes and vertical tubing with optimized rifledtubes [2]. Because of the increased supercritical main steamparameters of 700 !C at 350 bar the outlet temperature fromthe evaporator also increases with an equivalent release ofheat from the flue gas. Use of materials T23 and T24 withroughly 2.6 % chromium already implemented in evapora-tors would significantly limit the possible fuel spectrum in a700-!C power plant. Only coals with very high ash-soften-ing temperatures are used in order to limit the steam outlettemperature from a compact evaporator to approx. 500 !C.

A switch to the group of steels with 9 – 12 % chromiumcontent at least in parts of the evaporator was discussed inthe above projects and was considered a possible option.However, the cold drawing process for producing rifledtubes is limited to materials with a maximum chromiumcontent of roughly 5 %. The group of steels with 9 – 12 %chromium content is thus excluded for use as an evaporatortube in a Benson boiler with a low mass flux design –because rifled tubing is a prerequisite for this kind of evapo-rator tubing in a pulverized coal-fired boiler.

Following welding, the 9 – 12 % chromium steels requirepost-weld heat treatment, which is difficult to implement ina furnace with spiral tubing. The welds do not usually lie ina plane, and the necessary welds in the corners of the fur-nace result in a large number of points requiring heat treat-ment. For vertical tubing, the welds are always in a singleplane and the tubing does not have to be routed around thecorners in the furnace, resulting in advantages for the

Benson low mass flux design, especially regarding post-weldheat treatment.

For this reason, development was started for the produc-tion of a tube swirler insert in the context of R&D activitiesin the Benson license. The rotation of the fluid on the insideis to be generated by an insert and not by the cold-drawnrifling. The initial purpose of the development was to pro-duce a smooth tube with inserts providing very similar ther-mohydraulic characteristics to those of a highly optimizedrifled tube. Tests in the Benson test rig have shown thatwith optimized rifled tubing the design mass flux in anevaporator can be reduced to 50 % compared to a smoothtube for identical material temperatures.

In a rifled tube the boiling crisis does not take place untilsteam quality is > 0.9. The reason is the swirl flow generatedby the spiral ribs inside the tubes. Differences in centrifugalforce separate the water from the steam fraction and forcethe water to the tube wall. This maintains tube wall wettingup to high steam qualities, resulting in high flow velocitieseven at the location of the boiling crisis. This yields a goodheat transfer with low wall temperatures, especially in caseof two-phase flow.

In a so-called low mass flux design the pressure drop ofthe lower furnace tubing can be reduced, e.g., from 9 bar inan evaporator spiral with smooth tubes to roughly 2 bar in avertically tubed evaporator with optimized rifled tubing.The reduction in mass flux and pressure drop also leads toa change in flow characteristic. The pressure drop due tofriction is reduced relative to the hydrostatic pressure,changing the flow characteristic of the evaporator system tothat of natural circulation. Mass flow deviations caused bychanges in heat input are governed by the hydrostatic pres-

Chem. Ing. Tech. 2019, 91, No. 9, 1267–1272 ª 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cit-journal.com

–Martin Effert, Jan [email protected] AG, Gas and Power, GP EPC SO EN PP, Freyesleben-straße 1, 91058 Erlangen, Germany.

Research Article 1267ChemieIngenieurTechnik

sure drop. A higher heat input to a single tube results in anincrease in mass flow through this tube. The increased out-let temperature caused by excessive heat input is therebycompensated to the major part. The evaporator conceptwith low mass flux using optimized rifled tubing and thethermohydraulic behavior has been described in a numberof publications [3 – 8].

In many CFD (computational fluid dynamic) calcula-tions, the geometry of the insert was defined such that asimilar pressure drop and comparable internal heat transfercould be anticipated.

2 Insert Production

The key component for producing the inserts is what isknown as the template shaft. Wires are introduced in theslots in the template shaft, and these are inserted in asmooth tube together with the shaft (Fig. 1).

The wires of the subsequent insert are guided by thewinding head shown below to the template shaft and areclamped to the end face. If the template shaft is turnedclockwise in the winding head, it automatically takes alongthe wires guided by the winding head. The smooth heatexchange tube, into which the template shaft moves togeth-er with the insert, is positioned at the end face of the wind-ing head.

As soon as the shaft projects past the far end of the heatexchange tube, the rotation of the shaft is stopped. The wireclamp is loosened, and the shaft is then turned back slightly.When the shaft is turned back, the wires automatically stay

in their position in the tube. As soon as the template shaft isagain completely within the heat exchange tube, the projec-ting wires are welded to the end of the tube and cut off. Thetemplate shaft is then completely turned out of the heatexchange tube. The wires are also welded and then cut offon the end just ahead of the winding head.

Residual stresses cause the individual wires of the insertsto lie very securely against the inner wall of the tube. Fur-ther securing of the wires in the tubes is not necessary. Fig. 1shows the process on turning the template shaft back. Thefinished insert in the tube can be seen in front of the tem-plate shaft. This relatively simple setup has been used toincorporate inserts in tubes of up to 12 m in length with aninside diameter of 25.4 mm.

3 Results from Laboratory Tests

Since the mid-1970s, countless test series have been con-ducted in the Benson laboratory in Erlangen. Looking at thedesign data of the test rig as, e.g., a maximum pressure of330 bar, maximum fluid temperature of 600 !C, a mass flowof up to 4 kg s–1 for a single tube and a heating capacity of2000 kW, it is to our knowledge the world’s largest testfacility for measuring pressure drop and heat transfer in thetwo-phase regime [9, 10].

The test facility mainly comprises a water supply system,the object to be tested, a pressurizer and a cooling system(Fig. 2). In the water supply system, demineralized andde-aerated water or boiler feedwater to which chemicalshave been added to obtain the required water chemistry is

provided in a feedwater tank. This wateris injected into the test loop by a pistonpump. To minimize flow oscillationscaused by the pump’s six pistons, adamping vessel is installed in the pumpdischarge line.

The water is heated in a preheater anda main heater to establish the thermody-namic flow conditions required at theinlet to the test object. These coil-typeheaters are designed for direct electricheating. The advantage of this heatingmethod is that it allows precise controlof the heat added to the fluid. This isespecially important when developingheat transfer and pressure drop correla-tions. Experiments using other methodssuch as radiation heating are less suitablefor these tasks, because a precise deter-mination of fluid enthalpy or of actualheat input is more difficult.

After the fluid has passed through themain heater, it enters the test section,which is likewise equipped for directelectric heating, and where the flow and

www.cit-journal.com ª 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Ing. Tech. 2019, 91, No. 9, 1267–1272

Figure 1. Template shaft, winding head, tube and swirler insert on turning back (5-wireinserts).

1268 Research ArticleChemieIngenieurTechnik

heat transfer conditions are simulated and monitored.Installed downstream of the test object is a spray-type con-denser for condensing the steam fraction and subcoolingthe fluid. The subcooled fluid then passes to a circulationpump that recirculates part of the flow for cooling the spraycondenser. Condenser cooling water is taken from the mainflow immediately downstream of the pump and from themain cooler, which is supplied on its secondary side withwater from a wet cooling tower (trickle cooler).

The system pressure is adjusted by a large thermal pres-surizer and a throttling valve downstream of the test object.Assurance of a constant pressure is very important for stud-ies performed near the critical pressure, because pressurefluctuations can affect heat transfer conditions in the testobject.

The Benson test rig and the test object are instrumentedin such a way that all relevant parameters like temperature,pressure and flow can be measured. Data acquisition andprocess visualization play a key role in recording and moni-toring the individual experiments.

Inserts made by the method described above have beeninvestigated in several laboratory test series. The first testseries used an 8 m long smooth tube with an inside diame-ter of 25.4 mm and a 4-wire insert. Fig. 3 shows a piece ofthe 4-wire insert in a Plexiglas tube.

A 5-wire insert was used in the second testseries. In order to be able to use the alreadyexisting instrumentation, the insert was installedin the same smooth tube with 25.4 mm insidediameter and the entire test matrix was pro-cessed again. Fig. 4 shows the combinations ofmass flux, heat flux and pressure for the tests.The entire enthalpy range of interest is investi-gated at all pressures. At subcritical pressuresthis means the enthalpy range between subcool-ing at the inlet, the occurrence of the boiling cri-sis (departure from nucleate boiling (DNB) ordryout), and the single-phase steam flow at theoutlet. At supercritical pressures, a similarenthalpy range is studied throughout the entirepseudo-critical regime.

The key results of the heat transfer and pres-sure drop tests were that the 5-wire swirler insert

installed in the smooth tube exhibits excellent heat transferbehavior that is nearly identical to an optimized rifled tube.This applies especially for typical operating conditions in adesign with vertical tubes characterized by low mass flux.Fig. 5 shows a comparison of measured inner wall tempera-tures for the smooth tube with the 5-wire swirler insert withthose from tests with an optimized rifled tube (RR19) justbelow the critical pressure. A sharp increase in the tubeinner wall temperature results in a boiling crisis. Both thelocation of the boiling crisis as well as the wall temperaturesmeasured subsequently in the so-called post-CHF (post-critical heat flux) area are nearly identical.

Fig. 6 shows the measured inner wall temperatures of asmooth tube (DB04) compared with the swirler insert mea-surements in a comparable mass flux range. Looking at thetemperatures along the smooth tube with 300 kW m–2 heatflux, the boiling crisis DNB occurs slightly above an enthal-py of 1800 kJ kg–1, leading to an inside peak wall tempera-ture of 570 !C. With the swirler insert and more than doublethe heat flux, the peak wall temperature downstream of theboiling crisis was measured at 470 !C. This shows the mag-nitude of improvement in heat transfer by using swirlerinserts.

In addition, the tube friction factors in a smooth tubewith swirler inserts were very close to the values for an opti-mized rifled tube. The two development goals with regardto thermohydraulic properties were fully achieved using the5-wire inserts. Nevertheless, both, friction factors and innerheat transfer, can be further optimized as the shape of thewire as well as the lead angle can be varied in a much widerrange than in internally rifled tubes.

4 Cutting, Bending and Internal Weldingof the Insert

For possible use of the inserts in smooth tubes, such as inthe evaporator heat exchange surfaces of a fired boiler, it


Figure 2. Benson test rig.

Figure 3. 4-wire insert installed in a Plexiglas tube for demon-stration purposes.


must be possible to bend the tubes or also to cut out a tubesection from the evaporator wall and to replace it in theevent of a necessary repair.

After a tube with a swirler insert was cut, some of theindividual wires in an area up to a maximum of 15 cm from

the cut edge had lost their positions. However, the wireswere able to be easily moved back to their old position andfixed there by welding. Replacement of a defective tube sec-tion in a heat exchange surface would thus be possible withno difficulties. On bending the tubes, it proved that the


Figure 4. Test matrix.

Figure 5. Comparison of measured inner wall temperatures for smooth tube with 5-wire swirler insert withthose for an optimized rifled tube RR19 just below the critical pressure. Pressure 215 bar, mass flux 750 kg m–2s–1.


insert wires laid perfectly against the inside of the tube evenfor a small bend radius of 150 mm (just less than 4D), seeFig. 7.

To improve acceptance from potential customers, a pro-cedure was developed with which the insert can be weldedin the tube. The resistance welding method is used that isimplemented in areas including automotive manufactur-ing. A lance for welding in the tube was developed andconstructed based on the results obtained in preliminarytests (see Fig. 8). A hydraulic cylinder presses against theswirler insert on one inside and also presses the electrodeagainst the insert wire on the opposite side. The pressingforce can be adjusted via the pressure of the hydraulic flu-id. The current is routed through the insulated copperlance.

The welding lance described above was successfully usedto make welds on the insert in the tube. Automatic position-ing of the lance in the tube has not yet been developed, withthe result that the lance had to be optically aligned in theweld tests. Further development of the internal welding pro-cess, which is only regarded as an option, was discontinuedfor commercial reasons.

5 Outlook – Applications Within andOutside the Power Plant Industry

Initial discussions were conducted with a manufacturer ofrifled tubes for evaluation of the manufacturing process foran insert in comparison with cold-drawn rifled tubes. From


Figure 6. Comparison of measured inner wall temperatures for smooth tube with 5-wire swirler insert withthose for a smooth tube (DB04) just below the critical pressure. Pressure 215 bar, mass flux 750 kg m–2s–1 and715 kg m–2s–1 (DB04).

Figure 7. Insert in a tube bend.

Figure 8. Welding lance (without insulation).


the perspective of the tube manufacturer, it is not ruled outthat the production of an insert may be more cost-effectivethan the cold-drawing process. The resulting freedom ofmaterial selection and the option of also using differingmaterials for the tube and insert was evaluated as positive.

The industrial manufacturing process for inserts is cur-rently being investigated and evaluated regarding the costsin comparison with rifled tubes. The welding of the insertsin the tube that is seen as an option is not considered here.

As mentioned initially, development of the inserts wasstarted because a steel with a chromium content of 9 – 12 %must presumably be used for the evaporator tubing in aboiler with a high-pressure steam temperature of 700 !C forthe so-called Benson low mass flux design. A solution hasbeen developed with the inserts which can be used toreplace the cold-drawn rifling.

Development has now progressed to the point that, in thenext step, a test heat exchange surface would have to beproduced from smooth tubes with inserts and installed in aboiler. Such a test could demonstrate that the inserts pro-vide the necessary tube cooling in actual power plant opera-tion and that internal welding can be permanently fore-gone.

Unfortunately, the market has changed radically over thelast decade. The 700-!C projects are no longer under con-sideration in Europe and only Japan, China and India stillhave ongoing R&D activities in this area. It is uncertainwhether such a project will ever be implemented on an in-dustrial scale.

Implementation of the smooth tubes with inserts de-scribed in the article could be expedient in solar towerpower plants with direct evaporation. Stainless steels areused for the evaporator heat exchange surfaces due to theextremely high heat fluxes. The inserts significantly improveinternal heat transfer, especially in the two-phase zone, andreduce the material temperature of the heat exchange tube.Unfortunately, however, solar tower power plants have notyet achieved a significant market volume, and one has theimpression that photovoltaics are overtaking in the solarenergy field.

It is currently being tested and investigated whether thereare applications, such as in the chemical industry or in thepetrochemical industry, that could benefit from a significantimprovement in internal heat transfer through the use ofinserts. Internal heat transfer is especially improved whereflow is two-phase.

Development of the inserts is at a critical point. The origi-nally planned implementation in the evaporator of a firedboiler is becoming increasingly less probable. Future devel-opment to the point of production on an industrial scalewill only take place if further implementation possibilitiesare forthcoming.

Symbols used

h [kJ kg–1] enthalpyh’ [kJ kg–1] enthalpy of saturated waterh’’ [kJ kg–1] enthalpy of saturated steam

References

[1] www.vgb.org/en/research_project220.html[2] M. Effert, J. Bruckner, Benson low mass flux vertically-tubed

evaporators in the power market – a status update, in ModernPower Systems, April 2017.

[3] J. Franke, W. Kohler, E. Wittchow, VGB Kraftwerkstechn. 1993,73 (4), 352 – 361.

[4] J. Franke, R. Cossmann, H. Huschauer, VGB Kraftwerkstech.1995, 75 (4), 321 – 327.

[5] J. Franke, R. Kral, Operational Aspects and Performance of aBenson Boiler With Vertical Evaporator Tubing, Power-GenEurope, Amsterdam, May 1995.

[6] A. J. Bennett, M. J. Albrecht, C. S. Jones, Power-Gen Asia,Singapore, November 2010.

[7] S. J. Goidich, Power Gen International, Orlando, November 2000.[8] S. J. Goidich, R. J. Docherty, K. P. Melzer, Power Gen India &

Central Asia, New Delhi, May 2011.[9] www.siemens.com/benson

[10] W. Kastner, W. Kohler, H. Schmidt, VGB Kraftwerkstech. 2000,80 (6), 45 – 51.



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